It is well known that radio signals can suffer from a variety of influences that result in a radio receiver experiencing fading of the received radio signal. For example, a radio signal received at a receiver can be subject to Rayleigh fading, other multi-path, or environmental fading influences.
Rayleigh fading, and other multi-path fading effects, result when multipath versions of a signal destructively interfere to attenuate the signal arriving at a receiver. The fading experienced at a receiver typically is unique to that receiver, as it depends upon the particular multiple paths the signals travel between the transmitter and the receiver. Such fading is a significant problem in communications system, especially mobile systems where movement of the receiver and changes in the environment both contribute to changing multipath signals.
Prior attempts to deal with Rayleigh fading and the like have concentrated on providing redundant information in the signal via error correction coding, symbol repetition and interleaving of transmitted information, to allow a receiver to reconstruct symbols lost during a fade experienced at the receiver. While such techniques allow system designers to provide a selected probability of reception of a signal, this is achieved at a cost of reduced system throughput, as redundant information must be transmitted, reducing the bandwidth utilization efficiency of the transmission.
Various other approaches to dealing with Rayleigh fading and the like have also been proposed. For example, the third generation partnership project (“3GPP”), which is working to develop a next generation wireless communication system, has proposed a “compressed mode” of operation. Specifically, and as described in more detail in the documents available from the web site (www.3gpp.org) of the 3GPP organization, the 3GPP air interface has been defined as a slotted-frame architecture wherein data (voice data and “pure” data such as HTTP or FTP data) is typically transmitted from the base station to receivers in frames of ten millisecond duration and each frame comprises fifteen time slots of data. A receiver experiencing poor reception from a base station, due to a fade, or other factors, can inform the base station that it wishes to enter compressed mode for one or more of the next frames sent to it. Once the transmitter at the base station agrees, the agreed compressed frames are transmitted to the receiver.
Each compressed frame contains some agreed number of empty (i.e.—they do not contain data) slots at the end of the frame. The receiver, knowing that the agreed number of slots will not contain data intended for it, is free during the transmission time of those empty slots to attempt to receive signals from other transmitters, such as the pilot signal broadcast by each other base station, to evaluate the receiver's ability to receive those other transmitters. In other words, transmission of data to the receiver occurs only in a portion of the frame and nothing is transmitted in the balance of the frame when the receiver is not listening to that transmitter.
If the receiver can better receive another transmitter than the transmitter it has been listening to, the receiver can inform the current transmitter that it requires a hand off to the better received transmitter. In this manner, which can be considered a form of transmitter diversity, a receiver experiencing a fade or other poor reception characteristics from one transmitter can switch to another transmitter which it can receive at a better condition.
One problem with the 3GPP compressed mode is that radio transmission capacity is wasted whenever the empty slots are transmitted. It is presently contemplated by 3GPP that typically seven of the fifteen slots in a frame will be empty during compressed mode. Another problem with 3GPP compressed mode is that latency is increased during compressed mode as data which would otherwise be transmitted in the empty slots is delayed until a subsequent frame. Yet another problem with the 3GPP compressed mode is that it requires coordination between the transmitter and the receiver and this adds delay and overhead servicing requirements to the communication system. Thus, there will always be some additional delay in how quickly a receiver can evaluate its reception of other transmitters when it is experiencing a fade, or other poor reception characteristics, from the transmitter it is presently receiving. Also, compressed mode is only employed once poor reception is being experienced and it is not used proactively, to prevent unacceptable reception from occurring, where possible. Finally, 3GPP compressed mode presumes that another transmitter is available in the system and that a hand off of responsibility for the receiver can be achieved between the transmitters.